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  • 1.
    Demirel, Burak
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Architectures and Performance Analysis of Wireless Control Systems2015Doctoral thesis, monograph (Other academic)
    Abstract [en]

    Modern industrial control systems use a multitude of spatially distributed sensors and actuators to continuously monitor and control physical processes. Information exchange among control system components is traditionally done through physical wires. The need to physically wire sensors and actuators limits flexibility, scalability and reliability, since the cabling cost is high, cable connectors are prone to wear and tear, and connector failures can be hard to isolate. By replacing some of the cables with wireless communication networks, costs and risks of connector failures can be decreased, resulting in a more cost-efficient and reliable system.

    Integrating wireless communication into industrial control systems is challenging, since wireless communication channels introduce imperfections such as stochastic delays and information losses. These imperfections deteriorate the closed-loop control performance, and may even cause instability. In this thesis, we aim at developing design frameworks that take these imperfections into account and improve the performance of closed-loop control systems.

    The thesis first considers the joint design of packet forwarding policies and controllers for wireless control loops where sensor measurements are sent to the controller over an unreliable and energy-constrained multi-hop wireless network. For a fixed sampling rate of the sensor, the co-design problem separates into two well-defined and independent subproblems: transmission scheduling for maximizing the deadline-constrained reliability and optimal control under packet losses. We develop optimal and implementable solutions for these subproblems and show that the optimally co-designed system can be obtained efficiently.

    The thesis continues by examining event-triggered control systems that can help to reduce the energy consumption of the network by transmitting data less frequently. To this end, we consider a stochastic system where the communication between the controller and the actuator is triggered by a threshold-based rule. The communication is performed across an unreliable link that stochastically erases transmitted packets. As a partial protection against dropped packets, the controller sends a sequence of control commands to the actuator in each packet. These commands are stored in a buffer and applied sequentially until the next control packet arrives. We derive analytical expressions that quantify the trade-off between the communication cost and the control performance for this class of event-triggered control systems.

    The thesis finally proposes a supervisory control structure for wireless control systems with time-varying delays. The supervisor has access to a crude indicator of the overall network state, and we assume that individual upper and lower bounds on network time-delays can be associated to each value of the indicator. Based on this information, the supervisor triggers the most appropriate controller from a multi-controller unit. The performance of such a supervisory controller allows for improving the performance over a single robust controller. As the granularity of the network state measurements increases, the performance of the supervisory controller improves at the expense of increased computational complexity.

  • 2.
    Demirel, Burak
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Design and Performance Analysis of Wireless Networked Control Systems2013Licentiate thesis, monograph (Other academic)
    Abstract [en]

    Networked control systems (NCSs) are distributed systems that use shared communication networks to exchange information between system components such as sensors, controllers and actuators. The networked control system architecture promises advantages in terms of increased flexibility, reduced wiring and lower maintenance costs, and is finding its way into a wide variety of applications, ranging from automobiles and automated highway systems to process control, and power distribution systems. However, NCSs also pose many challenges in their analysis and design, since transmitting signals over wireless networks has several side effects, such as: (i) variable sampling intervals, (ii) variable communication delays, (iii) packet losses caused by the unreliability of the network. In this thesis, we aim at developing three different design frameworks, which take some of these side effects into account for improving the performance of the overall system.

    This thesis firstly presents the joint design of packet forwarding policies and controllers for wireless control loops where sensor measurements are sent to the controller over an unreliable and energy--constrained multi--hop wireless network. For fixed sampling rate of the sensor, the co--design problem separates into two well-defined and independent subproblems: transmission scheduling for maximizing the deadline--constrained reliability and optimal control under packet loss. We develop optimal and implementable solutions for these subproblems and show that the optimally co--designed system can be efficiently found. Numerical examples highlight the many trade-offs involved and demonstrate the power of our approach.

    Secondly, this thesis proposes a supervisory control structure for networked systems with time-varying delays. The control structure, in which a supervisor triggers the most appropriate controller from a multi-controller unit, aims at improving the closed-loop performance relative to what can be obtained using a single robust controller. Our analysis considers average dwell-time switching and is based on a novel multiple Lyapunov-Krasovskii functional. We develop stability conditions that can be verified by semi-definite programming, and show that the associated state feedback synthesis problem also can be solved using convex optimization tools. Extensions of the analysis and synthesis procedures to the case when the evolution of the delay mode is described by a Markov chain are also developed. Simulations on small- and large-scale networked control systems are used to illustrate the effectiveness of our approach.

    Lastly, we consider an event--triggered control framework for a linear time--invariant process. We introduce a range based event--triggering algorithm that is used to transmit information from the controller to the actuator. We also analytically characterize the control performance and communication rate for a given event threshold. Additionally, we provide a systematic way to analyze the trade--off between the communication rate and control performance by appropriately selecting an event threshold. Using numerical examples, we demonstrate the effectiveness of the proposed framework. 

  • 3.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Aytekin, Arda
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Quevedo, D. E.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    To wait or to drop: On the optimal number of retransmissions in wireless control2015In: 2015 European Control Conference, ECC 2015, IEEE , 2015, p. 962-968Conference paper (Refereed)
    Abstract [en]

    The dimensioning of wireless communication protocols for networked control involves a non-trivial trade-off between reliability and delay. Due to the lossy nature of wireless communications, there is a risk that sensor messages will be dropped. The end-to-end reliability can be improved by retransmitting dropped messages, but this comes at the expense of additional delays. In this work, we determine the number of retransmissions that strikes the optimal balance between communication reliability and delay, in the sense that it achieves the minimal expected linear-quadratic loss of the closed-loop system. An important feature of our setup is that it accounts for the random delays and possible losses that occur when unreliable communication is combatted with retransmissions. The resulting controller dynamically switches among a set of infinite-horizon linear-quadratic regulators, and is simple to implement. Numerical simulations are carried out to highlight the trade-off between reliability and delay. © 2015 EUCA.

  • 4.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Aytekin, Arda
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Quevedo, Daniel E.
    Univ Newcastle, Sch Elect Engn & Comp Sci, Callaghan, NSW 2308, Australia..
    Johansson, Mikael
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    To wait or to drop: on the optimal number of retransmissions in wireless control2015In: 2015 EUROPEAN CONTROL CONFERENCE (ECC), IEEE , 2015, p. 962-968Conference paper (Refereed)
    Abstract [en]

    The dimensioning of wireless communication protocols for networked control involves a non-trivial trade-off between reliability and delay. Due to the lossy nature of wireless communications, there is a risk that sensor messages will be dropped. The end-to-end reliability can be improved by retransmitting dropped messages, but this comes at the expense of additional delays. In this work, we determine the number of retransmissions that strikes the optimal balance between communication reliability and delay, in the sense that it achieves the minimal expected linear-quadratic loss of the closed-loop system. An important feature of our setup is that it accounts for the random delays and possible losses that occur when unreliable communication is combatted with retransmissions. The resulting controller dynamically switches among a set of infinite-horizon linear-quadratic regulators, and is simple to implement. Numerical simulations are carried out to highlight the trade-off between reliability and delay.

  • 5.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Briat, C.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Deterministic and stochastic approaches to supervisory control design for networked systems with time-varying communication delays2013In: Nonlinear Analysis: Hybrid Systems, ISSN 1751-570X, E-ISSN 1878-7460, Vol. 10, no 1, p. 94-110Article in journal (Refereed)
    Abstract [en]

    This paper proposes a supervisory control structure for networked systems with time-varying delays. The control structure, in which a supervisor triggers the most appropriate controller from a multi-controller unit, aims at improving the closed-loop performance relative to what can be obtained using a single robust controller. Our analysis considers average dwell-time switching and is based on a novel multiple Lyapunov-Krasovskii functional. We develop stability conditions that can be verified by semi-definite programming, and show that the associated state feedback synthesis problem also can be solved using convex optimization tools. Extensions of the analysis and synthesis procedures to the case when the evolution of the delay mode is described by a Markov chain are also developed. Simulations on small and large-scale networked control systems are used to illustrate the effectiveness of our approach.

  • 6.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Briat, Corentin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Supervisory control design for networked systems with time-varying communication delays2012In: Analysis and Design of Hybrid Systems, IFAC , 2012, p. 133-140Conference paper (Refereed)
    Abstract [en]

    This paper proposes a supervisory control structure for networked systems with time-varying delays. The control structure, in which a supervisor triggers the most appropriate controller from a multi-controller unit, aims at improving the closed-loop performance relative to what can be obtained using a single robust controller. Our analysis considers average dwelltime switching and is based on a novel multiple Lyapunov-Krasovskii functional. We develop analysis conditions that can be verified by semi-definite programming, and show that associated state feedback synthesis problem also can be solved using convex optimization. Small and large scale networked control systems are used to illustrate the effectiveness of our approach.

  • 7.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Feyzmahdavian, Hamid Reza
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Ghadimi, Euhanna
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Stability Analysis of Discrete-Time Linear Systems with Unbounded Stochastic Delays2015In: 5th IFAC Workshop on Distributed Estimation and Control of Networked Systems (NECSYS), Elsevier, 2015, Vol. 48Conference paper (Refereed)
    Abstract [en]

    This paper investigates the stability of discrete-time linear systems with stochastic delays. We assume that delays are modeled as random variables, which take values in integers with a certain probability. For the scalar case, we provide an analytical bound on the probability to guarantee the stability of linear systems. In the vector case, we derive a linear matrix inequality condition to compute the probability for ensuring the stability of closed-loop systems. As a special case, we also determine the step size of gradient algorithms with stochastic delays in the unconstrained quadratic programming to guarantee convergence to the optimal solution. Numerical examples are provided to show the effectiveness of the proposed analysis techniques.

  • 8.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Gupta, V.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    On the trade-off between control performance and communication cost for event-triggered control over lossy networks2013In: 2013 European Control Conference, ECC 2013, IEEE , 2013, p. 1168-1174Conference paper (Refereed)
    Abstract [en]

    This paper develops a theoretical framework for quantifying the trade-off between communication cost and control performance in event-triggered control over lossy networks. We consider a system where the communication between the controller and actuator is dictated by a threshold-based event-triggering algorithm, and develop a Markov-chain model that describes the attempted and successful transmissions of control messages over the lossy communication channel. A feature of our model is that it considers retransmissions of unsuccessful messages and that it accounts for the delay associated with such retransmissions. A systematic framework for analyzing the trade-off between the communication rate and control performance and for optimal tuning of the event threshold emanates by combining this model with an analytical model of the closed-loop performance. Numerical examples demonstrate the effectiveness of the proposed framework.

  • 9.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. University of Paderborn, Germany.
    Gupta, V.
    Quevedo, D. E.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    On the Trade-Off between Communication and Control Cost in Event-Triggered Dead-Beat Control2016In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. PP, no 99, article id 7562376Article in journal (Refereed)
    Abstract [en]

    We consider a stochastic system where the communication between the controller and the actuator is triggered by a thresholdbased rule. The communication is performed across an unreliable link that stochastically erases transmitted packets. To decrease the communication burden, and as a partial protection against dropped packets, the controller sends a sequence of control commands to the actuator in each packet. These commands are stored in a buffer and applied sequentially until the next control packet arrives. In this context, we study dead-beat control laws and compute the expected linear-quadratic loss of the closed-loop system for any given event-threshold. Furthermore, we provide analytical expressions that quantify the trade-off between the communication cost and the control performance of event-triggered control systems. Numerical examples demonstrate the effectiveness of the proposed technique.

  • 10.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. Univ Paderborn.
    Gupta, Vijay
    Quevedo, Daniel E.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Threshold Optimization of Event-Triggered Multi-Loop Control Systems2016In: 2016 13TH INTERNATIONAL WORKSHOP ON DISCRETE EVENT SYSTEMS (WODES), IEEE conference proceedings, 2016, p. 203-210Conference paper (Refereed)
    Abstract [en]

    This paper considers multiple linear stochastic control systems whose feedback loops are closed over a shared communication medium. A threshold-based event-triggering rule is used to transmit control commands from the controllers to the actuators, and network access is arbitrated using a static priority mechanism. Under these conditions, we study deadbeat control laws and compute the expected linear-quadratic loss of the closed-loop system as a function of the event-thresholds of the individual loops. Also, we present analytical expressions that quantify the trade-off between the communication cost and the control performance of such event-triggered control systems. Using a multi-dimensional exhaustive search method, we determine the set of event thresholds that attains the minimal expected linear-quadratic loss of the closed-loop systems. Simulation studies highlight the trade-off between the communication and control cost.

  • 11.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Guvenc, Levent
    Control of mechatronic systems - COMES toolbox2010In: PROCEEDINGS OF THE ASME 10TH BIENNIAL CONFERENCE ON ENGINEERING SYSTEMS DESIGN AND ANALYSIS, 2010, VOL 5, NEW YORK: AMER SOC MECHANICAL ENGINEERS , 2010, p. 233-246Conference paper (Refereed)
    Abstract [en]

    An interactive software tool based on MATLAB to analyze and design controllers for mechatronic systems is presented in this paper. This toolbox called COMES is a graphical user interface (GUI) to routines for four different control approaches: classical control (lead, lag, PID etc.), preview control, model regulator control and repetitive control. These control approaches have all found widespread use in the practical implementation of controllers for mechatronic systems. The aim is to design a user-friendly toolbox with a well designed graphical user interface (GUI), which hides all calculations from the user as much as possible. Thus, the user can focus on the design and analysis phases through the graphical displays rather than being burdened by the complicated calculations that are involved. The effectiveness of the use of this MATLAB-based toolbox was demonstrated by carrying out some design and simulation studies for several position control applications available in the literature.

  • 12.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Zou, Zhenhua
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Soldati, Pablo
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Modular Co-Design of Controllers and Transmission Schedules in WirelessHART2011In: IEEE Conference on Decision and Control, 2011, p. 5951-5958Conference paper (Refereed)
    Abstract [en]

    We consider the joint design of transmissionschedules and controllers for networked control loops that useWirelessHART communication for sensor and actuator data. Byparameterizing the design problem in terms of the samplingrate of the control loop, the co-design problem separates intotwo well-defined subproblems which admit optimal solutions:transmission scheduling should be done to maximize the delayconstrained reliability while the control design should optimizeclosed-loop performance under packet loss. We illustrate howthese problems can be solved and demonstrate our co-designframework for the case of linear-quadratic contro

  • 13.
    Demirel, Burak
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Zou, Zhenhua
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Soldati, Pablo
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Modular Design of Jointly Optimal Controllers and Forwarding Policies for Wireless Control2014In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 59, no 12, p. 3252-3265Article in journal (Refereed)
    Abstract [en]

    We consider the joint design of packet forwarding policies and controllers for wireless control loops where sensor measurements are sent to the controller over an unreliable and energy-constrained multi-hop wireless network. For fixed sampling rate of the sensor, the co-design problem separates into two well-defined and independent subproblems: transmission scheduling for maximizing the deadline-constrained reliability and optimal control under packet loss. We develop optimal and implementable solutions for these subproblems and show that the optimally co-designed system can be efficiently found. Numerical examples highlight the many trade-offs involved and demonstrate the power of our approach.

  • 14. Güvenç, B. A.
    et al.
    Necipoǧlu, S.
    Demirel, Burak
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Güvenç, L.
    Robust control of atomic force microscopy2013In: Mechatronics, John Wiley & Sons, 2013, p. 103-132Chapter in book (Other academic)
    Abstract [en]

    The atomic force microscope (AFM) is an instrument used for acquiring images at nanometer scale. Obtaining better image quality at higher scan speed is a research area of great interest in the control of an AFM. Improving the dynamic response of the scanning probe in the vertical direction and the dynamic response of the scanning motion in the lateral plane are the two major areas of application of advanced control methods to an AFM. The uncertainties inherent in the models of AFM vertical and lateral direction motion stages dictates the application of robust control methods. In this chapter, robust control methods are applied to AFM, treating first the vertical direction and then the lateral plane.

  • 15. Necipoǧlu, S.
    et al.
    Demirel, Burak
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Güvenç, L.
    Fast AFM scanning with parameter space based robust repetitive control designed using the comes toolbox2010In: ASME 2010 10th Biennial Conference on Engineering Systems Design and Analysis, ESDA2010: Volume 5, ASME Press, 2010, p. 599-606Conference paper (Refereed)
    Abstract [en]

    Atomic Force Microscope (AFM) is a very strong and beneficial instrument for acquiring images at nanometer scale. Hence, obtaining better image quality and scan speed is a research area of great interest. Improving the dynamic responses of the scanning probe and the vertical motion of the scanner mechanisms are the two major areas of concentration in this sense. Improving the vertical dynamics is achieved either by designing more complex scanner mechanisms with higher bandwidth or designing more sophisticated controllers rather than the PI, PID or PIID types of controllers that are mostly used in practice. In this paper, the authors focus on designing a repetitive control scheme for fast and accurate scanning. It is possible to implement repetitive control to achieve this goal when it is considered that the successive lines of the scan are quite similar due to the very small steps taken to advance on the sample. Repetitive control can reject higher frequency disturbances due to the surface topography in AFM much better than a conventional controller can, as it can drive the error caused by any periodic input signal to zero. Besides increasing the scan speed, it is also important that the phase lag can be compensated perfectly using repetitive control, with the knowledge of the surface topography from the previous period by introducing appropriate phase advance.

  • 16.
    Zou, Zhenhua
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Demirel, Burak
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Minimum-energy packet forwarding policies for guaranteed LQG performance in wireless control systems2012In: 2012 IEEE 51st Annual Conference on Decision and Control (CDC), IEEE , 2012, p. 3341-3346Conference paper (Refereed)
    Abstract [en]

    This paper studies minimum-energy packet forwarding policies for communicating sensor measurements from plant to controller over an unreliable multi-hop wireless network so as to guarantee that the optimal controller achieves a prespecified closed-loop performance. For fixed sampling interval, we demonstrate that the minimal linear-quadratic control loss is monotonically decreasing in the reliability of the sensor-to-controller communication. This allows us to decompose the overall design problem into two separate tasks: finding the minimum end-to-end reliability that allows to achieve a prespecified linear-quadratic loss, and developing minimum-energy packet forwarding policies under a deadline-constrained reliability requirement. We develop optimal solutions for both subproblems and show how the co-designed system with minimum forwarding energy cost and guaranteed LQG control performance can be found by a one-dimensional search over admissible sampling periods. The paper ends with a numerical example which demonstrates the effectiveness of the proposed framework.

1 - 16 of 16
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